Islanding effect is self power supply islanding phenomenon unable to be controlled by the power company where a distributed power generation system supplies power to surrounding loads, which is formed when power transmission and distribution operation ceases in the power supply system of the power company because of reasons such as faulty accident and outage maintenance, and the distributed power generation system installed at each user end fails to detect the condition of outage timely and therefore cannot isolate itself from the main electricity network. Islanding effect may adversely affect the equipment of an entire power distribution system and of user end, and associated personnel, including:
1) harming the life security of the power maintenance personnel;
2) affecting the protection switch action procedures in the power distribution system;
3) damaging appliances due to the unstable property of voltage and frequency of power supply in the islanding area;
4) re-tripping or damaging photovoltaic generation system, load, and power supply system which may be caused by the inrush current generated by voltage phase being non-synchronous when the power supply restores; and
5) inducing lack phase supply issues in three-phase load of system due to single-phase supply in the distributed generation system.
Existing methods for detecting islanding mainly include method of passive detection and method of active detection.
Method of passive detection: the passive method utilizes variation of voltage, frequency, phase, or harmonic at the output terminal of inverters when the power interrupts in the power grid for islanding effect detection. However, the method of passive detection will lose its ability to detect islanding effect under the situation where the output power of the distributed generation system is balanced with local load power. Also, the fundamental principle of this method is to monitor the data varying rate and determine if the data varying rate surpasses a pre-set threshold value, therefore there is a relatively large non-detection zone (NDZ). Consequently, the degree of accuracy and sensitivity of the detection result is not high.
Method of active detection: the active method causes certain disturbance of output power, frequency, or phase of the inverter by controlling the inverter. When the power grid operates normally, such disturbance is not detected due to balance action of the power grid. Once failure exists in the power grid, the disturbance output from the inverter will be accumulated rapidly and surpass the allowable range so that the islanding effect detection circuit will be triggered. However, this method has complicated control circuits and lowers the energy quality output from the inverter, thus the energy quality on user side will be affected significantly.
Therefore, a necessity exists for a method and system for detecting islanding effect with higher degree of accuracy and sensitivity and without compromising the energy quality on user side.